Aircraft fuel tank flammability reduction methods and systems and air separation methods using membranes

ABSTRACT

An aircraft fuel tank flammability reduction method includes contacting a membrane filter with air feed, permeating oxygen and nitrogen from the air feed through the membrane, and producing filtered air from the filter. The filtered air is produced from the filter as a result of the membrane removing any hydrocarbons containing six or more carbon atoms to produce a total of 0.001 ppm w/w or less. An air separation method includes feeding air into a filter containing a hollow fiber membrane that exhibits the property of resisting degradation due to exposure to hydrocarbons containing six or more carbon atoms. The filter exhibits a pressure drop across the membrane of less than 5 psi. The method includes feeding the filtered air into an air separation module containing a hollow fiber membrane that exhibits a susceptibility to degradation from exposure to hydrocarbons containing six or more carbon atoms.

TECHNICAL FIELD

The embodiments relate to methods and systems for reducing flammabilityin aircraft fuel tanks using membranes and air separation methods usingmembranes.

BACKGROUND

A variety of known systems exist with the purpose of reducingflammability in aircraft fuel tanks. Such systems may be known by anumber of designations including, but not limited to, On-Board Inert GasGeneration System (OBIGGS), Nitrogen Generation System (NGS),Flammability Reduction System (FRS), Fuel Tank Inerting System (FTIS),etc. However, a commonality among the systems involves reducing theoxygen content of fuel tank ullage by feeding inert gas into the fueltank. Often, the systems produce nitrogen-enriched air (NEA) for theinert gas. Air with lower percent oxygen is less flammable.

Inerting systems used to produce nitrogen-enriched air may rely onpressure swing absorption and desorption from media as a separationmechanism or diffusion through polymer membranes as another separationmechanism to remove oxygen. In systems with polymer hollow fibermembranes, compressed air enters the bore of the polymer hollow fiberand oxygen permeates through the polymer hollow fiber walls. The oxygenpermeate is collected and exhausted overboard. The remainingnitrogen-enriched retentate flows through the bore and is collected atthe air separation module product gas outlet for distribution toaircraft fuel tanks. Unfortunately, service life of the air separationmodule and the system operating conditions may be limited by thepolymers used in construction of the gas separation module. Accordingly,increased reliability of air separation modules is desirable.

SUMMARY

In an embodiment, an aircraft fuel tank flammability reduction methodincludes feeding pressurized air into a filter containing a membrane,contacting the membrane with the air feed, permeating oxygen andnitrogen from the air feed through the membrane, and producing filteredair from the filter. Contaminants in the pressurized air includehydrocarbons containing six or more carbon atoms. The filtered air isproduced from the filter as a result of the membrane removing anyhydrocarbons containing six or more carbon atoms to produce a total of0.001 parts per million by weight/weight (ppm w/w) or less. The methodincludes feeding the filtered air into an air separation module andproducing nitrogen-enriched air from the air separation module. Thenitrogen-enriched air is fed into the fuel tank on board the aircraft.

In another embodiment, an air separation method includes feedingpressurized air into a filter containing a hollow fiber membrane,contacting the hollow fiber membrane with the air feed, permeatingoxygen and nitrogen from the air feed through the membrane and producingfiltered air from the filter. Contaminants in the pressurized airinclude hydrocarbons containing six or more carbon atoms. The hollowfiber membrane exhibits the property of resisting degradation due toexposure to the hydrocarbons. The filtered air is produced from thefilter as a result of the membrane removing hydrocarbons containing sixor more carbon atoms. Further, the filter exhibits a pressure dropacross the membrane of less than 5 psi. The method includes feeding thefiltered air into an air separation module containing a hollow fibermembrane and producing nitrogen-enriched air from the air separationmodule. The ASM hollow fiber membrane exhibits a susceptibility todegradation from exposure to hydrocarbons containing six or more carbonatoms.

In a further embodiment, an aircraft fuel tank flammability reductionsystem includes a source for air, a filter configured to receive airfeed from the air source, and a membrane in the filter. The membrane isconfigured to permeate oxygen and nitrogen from the air feed through themembrane at a pressure drop across the membrane of less than 5 psi andto produce filtered air from the filter as a result of the membraneremoving hydrocarbons containing six or more carbon atoms. The systemincludes an air separation module configured to receive filtered airfrom the filter and to produce nitrogen-enriched air from the airseparation module. A fuel tank on board the aircraft is configured toreceive the nitrogen-enriched air.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments or may be combined in yetother embodiments further details of which can be seen with reference tothe following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments are described below with reference to the followingaccompanying drawings.

FIGS. 1-3 show diagrams of fuel tank flammability reduction systemsaccording to several embodiments.

DETAILED DESCRIPTION

Known aircraft fuel tank flammability reduction systems include apressurized air source, an air separation module (ASM) configured toreceive air feed from the pressurized air source, and a fuel tank onboard the aircraft configured to receive nitrogen-enriched air from theair separation module. Careful observation and evaluation has shown thatknown pressurized air sources available on aircraft, such as enginebleed air, may be contaminated with various gases (including hydrocarbongases) and liquid or solid aerosols of various sizes. Larger particlesmay also be present. More particularly, engine bleed air has beendemonstrated to contain residue and degradation products from jet fuel,engine lubricating oil, hydraulic fluid, de-icing agents, and othercontaminants present in the atmosphere, on the ground, and at altitude.Predominant contaminants are hydrocarbons containing only hydrogen andcarbon, but other hydrocarbons and other contaminants, such asaldehydes, ketones, acids, and other gases may be present. Gasseparation membranes, in general, are very susceptible to largehydrocarbon molecules, degradation products of which were further shownto contain six or more carbon atoms.

Air separation modules (ASMs) known for use in aerospace contain hollowfiber membranes, which permeate oxygen through the membranepreferentially to nitrogen. The molecules that do not permeate areretained (retentate) and are called nitrogen-enriched air. However, inoperating environments, ASMs exhibit loss of performance due tocontamination and due to the natural relaxation for the fiber. In somecases, ASMs exhibit decreased service life. Contaminants can negativelyaffect the polymer performance and life in several ways. Fiber pores canbe plugged by particulates. Liquids can coat membranes (form a boundarylayer), cause polymer swelling, or destroy membrane integrity. Polymersolvents could contribute to delamination of a polymer separation layeror within the separation layer and could lead to compaction (increase inseparation layer thickness) or fiber deformation. Gasses can fill upfree volume or, in significant levels, slowly accumulate to the membranesurface, decreasing permeation rate (especially heavy hydrocarbons withmore than 15 carbon atoms). Gasses can cause plasticization oranti-plasticization at elevated concentrations or can reduce molecularweight of the polymer (break polymer chains). Additionally, polymermaterials used to form hollow fiber membranes and other membranes mayexhibit a susceptibility to degradation due to exposure to thehydrocarbons containing six or more carbon atoms.

Known aircraft fuel tank flammability reduction systems may include aparticulate filter in an attempt to remove particles and/or includeanother filter, such as a liquid aerosols filter. However, known filtersupstream of an ASM are not known to remove hydrocarbons containing sixor more carbon atoms or small liquid or solid aerosols.

To maximize the available pressure and minimize system weight andmaintenance, known aerospace systems utilize liquid and particulatefiltration and account for the performance drop due to gaseouscontamination (other than ozone) in the system design. Accordingly, itis not known for a filter upstream of an ASM to remove hydrocarbonscontaining six or more carbon atoms or small liquid or solid particlesand also to exhibit a pressure drop of less than 5 pounds/inch² (psi).In a related manner, it is not known for such a filter to exhibit a highpermeability.

Further, although hollow fiber membranes are known for use in an ASM forseparation of oxygen from air, they are not known for use in othercomponents of an aircraft fuel tank flammability reduction system, suchas in a filter. It follows that a hollow fiber membrane in the ASMsusceptible to contaminants received from the pressurized air sourcewould also be susceptible to contaminants from the pressurized airsource when functioning as a filter. Such a membrane in a filter mayexhibit the same limited service life observed in the ASM. However,advances in material science for membranes functioning in applicationsother than aerospace show promise in exhibiting characteristics suitablefor use in the filter upstream of an ASM.

Specifically, new materials may resist degradation due to exposure tohydrocarbons containing six or more carbon atoms. Although suchmaterials might not function to effectively remove oxygen from air, theymay function effectively as a membrane filter removing small liquid orsolid particles and hydrocarbons containing six or more carbon atoms.

Consequently, a known material may be selected for use as a membrane inan ASM and designed to effectively remove oxygen from air. Examples ofpotentially suitable known polymers for such materials includepolyphenylene oxide (PPO), polyimide, polysulfone, polycarbonate, andothers, such as described in U.S. Pat. No. 8,245,978 issued to Beers andU.S. Pat. No. 7,699,911 issued to Zhou. Additionally, a materialdifferent from that of the ASM membrane may be used in a filter upstreamof the ASM as a membrane to effectively remove contaminants.Accordingly, the membrane in the filter may be less susceptible todegradation from exposure to hydrocarbons containing six or more carbonatoms compared to the membrane in the ASM. Even so, the membrane in theASM may be more effective in removing oxygen from air compared to themembrane in the filter. The different material in the membrane filtermight not be previously known for such use. Contaminants remaining inthe retentate of the membrane filter may be collected for some later useor vented, either alone or along with the permeate (oxygen) from theASM.

Although the embodiments herein are discussed as significant in usealong with a hollow fiber membrane ASM, they may also have applicabilityto other gas separation technologies. Also, although discussed herein inthe context of aircraft fuel tank flammability reduction systems, othergas separation systems may benefit from the concepts in the describedembodiments.

The described membrane filter may be placed downstream of a knownfilter. The described membrane filter may benefit from the removal ofparticles and/or liquid aerosols performed by a known filter. Servicelife of the membrane filter may thus be increased if positioneddownstream of a known particulate and/or liquid aerosols filter.Additionally, or instead of using a known filter, the membrane filtermay incorporate a sweep gas feature to assist in clearing the membraneof accumulated contaminants, such as is generally known.

Using one or more of the embodiments described herein, the service lifeof an ASM may be extended and system performance may increase bylimiting membrane performance degradation due to gaseous contamination.Accordingly, ASMs may be sized smaller, saving weight and space.Currently, an ASM is often sized based on an end-of-life performancethat accounts for performance degradation over time. With decreaseddegradation to the membrane due to contaminants described herein, agiven surface area available for permeating oxygen may be maintained fora longer time. The longer life may decrease the surface area needed toreach the same service life as desired without the embodiments herein.Alternately, the same surface area may be used and an extended servicelife realized.

In known, non-aerospace applications, multiple filters may be staged toprovide effective removal of contaminants upstream of an air separationsystem. The multiple filters add to system cost and maintenance time andmay be eliminated or reduced in number relying on the embodimentsherein. In non-aerospace applications, activated carbon is known for useas an adsorbent to remove unwanted hydrocarbons from an air source.However, activated carbon is considered unsuitable for use in aerospaceapplications given the need for regeneration and/or additional airplanemaintenance cost of activated carbon filtration replacement.Additionally, membrane feed pressure may decrease due to the pressuredrop through the activated carbon filter, which negatively impacts gasseparation membrane performance. More weight and volume of activatedcarbon could be used to allow hydrocarbon removal without frequentregeneration and/or replacement. Accordingly, unless a large volume ofactivated carbon is provided, servicing and usefulness of the activatedcarbon as a hydrocarbon removal medium is severely limited.

As a result, instead of focusing on new materials to replace membranesin ASMs, the embodiments herein take the approach of retaining knowntechnology with membrane materials susceptible to contaminants, buteffective for air separation. Known technology may be combined withmaterials as a membrane filter unsuitable for O₂/N₂ separation (such as,high O₂ and N₂ permeability and low selectivity), but focused on notpermeating higher molecular weight contaminants. The benefits ofmembrane technology may be achieved in both respects.

In an embodiment, an aircraft fuel tank flammability reduction methodincludes feeding pressurized air into a filter containing a membrane,contacting the membrane with the air feed, permeating oxygen andnitrogen from the air feed through the membrane, and producing filteredair from the filter. Contaminants in the pressurized air includehydrocarbons containing six or more carbon atoms. The filtered air isproduced from the filter as a result of the membrane removing anyhydrocarbons containing six or more carbon atoms to produce a total of0.001 parts per million by weight/weight (ppm w/w) or less. The methodincludes feeding the filtered air into an air separation module andproducing nitrogen-enriched air from the air separation module. Thenitrogen-enriched air is fed into the fuel tank on board the aircraft.

By way of example, the filter may exhibit a pressure drop across themembrane of less than 5 psi. The membrane may exhibit the property ofresisting degradation due to exposure to the hydrocarbons containing sixor more carbon atoms. As one option, the membrane may include a hollowfiber membrane, which may be polymer-based. The ASM may also include ahollow fiber membrane. The hollow fiber membrane of the ASM may exhibita susceptibility to degradation due to exposure to the hydrocarbonscontaining six or more carbon atoms. The method may further includeoperating a particulate filter that lacks a membrane upstream of thefilter containing the membrane.

The susceptibility to degradation may decrease permeability due togaseous contamination (other than ozone) and may vary by polymer. Thehigher the free volume of the polymer, the higher the performance, butalso the higher the permeability drop due to contaminants because itincludes more free volume to occupy. Using membrane filtration mayincrease the practicability of certain polymers in the ASM that wouldotherwise experience a permeability drop of about 20% or greater due tonatural relaxation of the fiber and gaseous contamination. Without thedescribed embodiments that include membrane filtration, such polymersmay exhibit a high enough performance drop over the service life that itmay not be practical to account for permeability loss in system sizing.Membrane polymers with a permeability drop that could practically beaccounted for in the system design may still benefit from membranefiltration as discussed herein because the lower drop would positivelyaffect the system component sizing.

In another embodiment, an air separation method includes feedingpressurized air into a filter containing a hollow fiber membrane,contacting the hollow fiber membrane with the air feed, permeatingoxygen and nitrogen from the air feed through the membrane and producingfiltered air from the filter. Contaminants in the pressurized airinclude hydrocarbons containing six or more carbon atoms. The hollowfiber membrane exhibits the property of resisting degradation due toexposure to the hydrocarbons. The filtered air is produced from thefilter as a result of the membrane removing hydrocarbons containing sixor more carbon atoms. Further, the filter exhibits a pressure dropacross the membrane of less than 5 psi. The method includes feeding thefiltered air into an air separation module containing a hollow fibermembrane and producing nitrogen-enriched air from the air separationmodule. The ASM hollow fiber membrane exhibits a susceptibility todegradation from exposure to hydrocarbons containing six or more carbonatoms.

By way of example, the filter membrane removes any hydrocarbonscontaining six or more carbon atoms to produce a total of 0.001 ppm w/wor less. Also, the method may include reducing aircraft fuel tankflammability using the nitrogen-enriched air.

In a further embodiment, an aircraft fuel tank flammability reductionsystem includes a source for air, a filter configured to receive airfeed from the air source, and a membrane in the filter. The membrane isconfigured to permeate oxygen and nitrogen from the air feed through themembrane at a pressure drop across the membrane of less than 5 psi andto produce filtered air from the filter as a result of the membraneremoving hydrocarbons containing six or more carbon atoms. The systemincludes an air separation module configured to receive filtered airfrom the filter and to produce nitrogen-enriched air from the airseparation module. A fuel tank on board the aircraft is configured toreceive the nitrogen-enriched air.

By way of example, the air source may be configured to providepressurized air. The membrane may be configured to remove anyhydrocarbons containing six or more carbon atoms to produce a total of0.001 ppm w/w or less. The membrane may exhibit the property ofresisting degradation due to exposure to the hydrocarbons containing sixor more carbon atoms. As one option, the membrane may include a hollowfiber membrane, which may be polymer-based. The ASM may also include ahollow fiber membrane. The air separation module may include a hollowfiber membrane exhibiting a susceptibility to degradation from exposureto hydrocarbons containing six or more carbon atoms. The system mayfurther include a particulate filter that lacks a membrane upstream ofthe filter containing the membrane.

FIG. 1 shows a diagram of a fuel tank flammability reduction system 10.In system 10, a pressurized air source 16 provides air feed 17 to amembrane filter 22. Membrane filter 22 produces filtered air 21 andretentate gas 24, containing contaminants removed from air feed 17.Membrane filter 22 may remove any hydrocarbons containing six or morecarbon atoms to produce a total of 0.001 ppm w/w or less. Also, membranefilter 22 may exhibit a pressure drop across its membrane of less than 5psi. Further, its membrane may exhibit the property of resistingdegradation due to exposure to the hydrocarbons containing six or morecarbon atoms. As an example, the membrane may be a hollow fibermembrane.

A downstream air separation module 12 receives filtered air 21 andproduces nitrogen-enriched air 19 along with permeate gas 18. Airseparation module 12 may include a hollow fiber membrane. The membranemay exhibit a susceptibility to degradation from exposure tohydrocarbons containing six or more carbon atoms. Given the removal ofcontaminants in retentate gas 24 by membrane filter 22, air separationmodule 12 is enabled to more effectively permeate oxygen through amembrane (not shown) and into permeate gas 18. Nitrogen-enriched air 19is provided to a fuel tank 14 for flammability reduction.

FIG. 2 shows a diagram of a fuel tank flammability reduction system 20that includes all the elements of system 10, but further includes aparticulate filter 26. Although not shown, particulate filter 26 mayadditionally function as a liquid aerosols filter or a separate liquidaerosols filter may be added to system 20 upstream or downstream ofparticulate filter 26. Particulate filter 26 provides filtered air 23 tomembrane filter 22. In system 20, particulate filter 26 may prolong theservice life of membrane filter 22 by removing contaminants such aslarge particles and liquid aerosols that may limit the effective surfacearea of the membrane (not shown) in membrane filter 22.

FIG. 3 shows a diagram of an aircraft fuel tank flammability reductionsystem 30 that includes all the elements of system 10 shown in FIG. 1,but additionally includes a heat exchanger 32. Often, known sources forpressurized air source 16 provides air feed 17 at an elevatedtemperature that may be unsuitable for the membrane in air separationmodule 12 and/or the membrane in membrane filter 22. Heat exchanger 32may be used to produce cooled air feed 34 to reduce heat damage tomembranes in downstream components. Alternatively, it is conceivablethat pressurized air source 16 may provide air feed 17 at a temperaturelimiting the performance of membranes in downstream components becauseit is too cold. In such case, heat exchanger 32 may instead produce aheated air feed (not shown). It is further conceivable that the membranein membrane filter 22 and the membrane in air separation module 12 mightoperate most efficiently in different temperature ranges. Accordingly,heat exchanger 32 may instead be positioned between membrane filter 22and air separation module 12 or an additional heat exchanger may beprovided to satisfy the temperature ranges of respective membranes. Evenfurther, conceivably membrane filter 22 and air separation module 12 mayinclude membranes operable at temperatures such that heat exchanger 32may instead be located downstream of air separation module to coolnitrogen-enriched air 19 before being provided to fuel tank 14.

Although systems 10, 20, and 30 discussed above each include fuel tank14, it is noted consistent with the discussion above thatnitrogen-enriched air 19 may instead be provided to a differentcomponent of a different system, such as an air separation system.Although FIGS. 1-3 show various possible embodiments of systemsdescribed herein, it will be appreciated that further combinations ofthe features in FIGS. 1-3 and other features described herein arecontemplated.

In compliance with the statute, the embodiments have been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the embodiments are not limited tothe specific features shown and described. The embodiments are,therefore, claimed in any of their forms or modifications within theproper scope of the appended claims appropriately interpreted inaccordance with the doctrine of equivalents.

TABLE OF REFERENCE NUMERALS FOR FIGURES 10 system 12 air separationmodule 14 fuel tank 16 pressurized air source 17 air feed 18 permeategas 19 nitrogen-enriched air 20 system 21 filtered air 22 membranefilter 23 filtered air 24 retentate gas 26 particulate filter 30 system32 heat exchanger 34 cooled air feed

The invention claimed is:
 1. An aircraft fuel tank flammabilityreduction method comprising: feeding pressurized air into a filtercontaining a membrane, contaminants in the pressurized air includinghydrocarbons containing six or more carbon atoms; contacting themembrane with the air feed, permeating oxygen and nitrogen from the airfeed through the membrane, and producing filtered air from the filter asa result of the membrane removing any hydrocarbons containing six ormore carbon atoms to produce a total of 0.001 ppm w/w or less; feedingthe filtered air into an air separation module (ASM) and producingnitrogen-enriched air from the air separation module; and feeding thenitrogen-enriched air into the fuel tank on board the aircraft.
 2. Themethod of claim 1 wherein the filter exhibits a pressure drop across themembrane of less than 5 pounds/inch² (psi).
 3. The method of claim 1wherein the membrane exhibits a property of resisting degradation due toexposure to the hydrocarbons containing six or more carbon atoms.
 4. Themethod of claim 1 wherein the membrane comprises a hollow fibermembrane.
 5. The method of claim 1 wherein the ASM comprises a hollowfiber membrane.
 6. The method of claim 5 wherein the ASM hollow fibermembrane exhibits a susceptibility to degradation from exposure tohydrocarbons containing six or more carbon atoms.
 7. The method of claim1 further comprising operating a particulate filter that lacks amembrane upstream of the filter containing a membrane.
 8. An airseparation method comprising: feeding pressurized air into a filtercontaining a hollow fiber membrane, contaminants in the pressurized airincluding hydrocarbons containing six or more carbon atoms and thehollow fiber membrane exhibiting a property of resisting degradation dueto exposure to the hydrocarbons; contacting the hollow fiber membranewith the air feed, permeating oxygen and nitrogen from the air feedthrough the membrane, and producing filtered air from the filter as aresult of the membrane removing hydrocarbons containing six or morecarbon atoms, the filter exhibiting a pressure drop across the membraneof less than 5 pounds/inch² (psi); and feeding the filtered air into anair separation module (ASM) containing a hollow fiber membrane andproducing nitrogen-enriched air from the air separation module, the ASMhollow fiber membrane exhibits a susceptibility to degradation fromexposure to hydrocarbons containing six or more carbon atoms.
 9. Themethod of claim 8 wherein the filter membrane removes any hydrocarbonscontaining six or more carbon atoms to produce a total of 0.001 ppm w/wor less.
 10. The method of claim 8 further comprising reducing aircraftfuel tank flammability using the nitrogen-enriched air.
 11. An aircraftfuel tank flammability reduction system comprising: a source for air; afilter configured to receive air feed from the air source; a membrane inthe filter, the membrane being configured to permeate oxygen andnitrogen from the air feed through the membrane at a pressure dropacross the membrane of less than 5 pounds/inch² (psi) and to producefiltered air from the filter as a result of the membrane removinghydrocarbons containing six or more carbon atoms; an air separationmodule (ASM) configured to receive filtered air from the filter and toproduce nitrogen-enriched air from the air separation module; and a fueltank on board the aircraft and configured to receive thenitrogen-enriched air.
 12. The system of claim 11 wherein the air sourceis configured to provide pressurized air.
 13. The system of claim 11wherein the membrane is configured to remove any hydrocarbons containingsix or more carbon atoms to produce a total of 0.001 ppm w/w or less.14. The system of claim 11 wherein the membrane exhibits a property ofresisting degradation due to exposure to the hydrocarbons containing sixor more carbon atoms.
 15. The system of claim 11 wherein the membranecomprises a hollow fiber membrane.
 16. The system of claim 11 whereinthe ASM comprises a hollow fiber membrane.
 17. The system of claim 16wherein the ASM hollow fiber membrane exhibits a susceptibility todegradation from exposure to hydrocarbons containing six or more carbonatoms.
 18. The system of claim 11 further comprising a particulatefilter that lacks a membrane upstream of the filter containing themembrane.